Etalon cavity with filler layer for thermal tuning

ABSTRACT

A temperature-stable and temperature-tunable composite etalon with an increased field of view is constructed by partially filling the cavity with a transmissive parallel-plate filler. The coefficient of thermal expansion and the index of refraction of the filler material are selected so as to produce the desired rate of change of the optical length of the cavity as a function of temperature. The filler plate is preferably chosen to be significantly thicker than the remaining air gap in the etalon cavity. As a result, the fact that the filler plate occupies a large part of the cavity space increases the acceptance angle of the etalon of the invention in comparison with corresponding conventional air-spaced etalons.

RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.10/841,131, filed May 7, 2004, which is based on U.S. Provisional Ser.No. 60/512,050, filed Oct. 17, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the general field of optical filters and, inparticular, to temperature-stable and tunable high-performance etalonfilters.

2. Description of the Prior Art

Etalons are well known optical devices that consist of two reflectivesurfaces parallel to one another and spaced apart by solid spacers toproduce a predetermined optical length (the “cavity length”). They mayconsists simply of a solid parallel plate with reflective surfaces (socalled “solid etalons”) or of two plates with an air gap between themthat defines the cavity (so called “air-spaced etalons”), as illustratedin FIG. 1. A hybrid form of etalon (so called “re-entrant etalon”)utilizes an additional solid structure with a reflective surface (a“riser”) between the two plates in order to achieve narrower cavitylengths than practically obtainable with the use of spacers.

When illuminated with a broadband collimated light, etalons produce atransmission beam and a reflection beam with periodic spectracharacterized by very narrowband spikes centered at wavelengthdetermined by the physical properties and dimensions of the etalon. Atypical etalon transmission spectrum is illustrated in FIG. 2. Withreference to air-spaced etalons, in particular, the specific centerwavelength λ′ of the passband (the spectral spike) and the periodbetween spectral spikes (commonly referred to in the art as channelspacing or free spectral range—FSR—of the device) are a function of theoptical length of the etalon's cavity. This disclosure is limited to adiscussion of transmission operation because those skilled in the artwould readily understand that it is similarly applicable to reflectionoperation.

In particular, referring for example to the etalon 10 and the intensityspectrum 12 of FIGS. 1 and 2, respectively, minor changes in the opticallength L (corresponding to the geometric length L′ shown in the figures)of the cavity 14 will cause a shift of the periodic spectrum along thewavelength axis, as indicated by arrows 16. As is well understood bythose skilled in the art, varying the optical length L of the cavityalso produces a change in the width of the spectral spike and in thefree spectral range of the etalon.

These properties of etalons are very advantageous for many opticalapplications. In particular, etalons are used as high-performancefilters to isolate light of a very a precise frequency, as may be neededfor a particular application. In telescopic astronomy, for instance,such filters are particularly useful for observing objects at specificwavelengths. Since the exact wavelength of each peak is a function ofthe exact optical length L of the cavity, it has been most important inthe art to build etalon filters with precise and uniform spacing betweenthe two plates (18,20) constituting the etalon (FIG. 1). To that end,very precisely machined spacers 22,24 of equal thickness L′ are used,typically uniformly distributed around the annular periphery of theplates in a sufficient number to separate the plates and produce acavity of uniform optical length L. Moreover, these spacers aretypically made of materials having a low coefficient of thermalexpansion. (It is noted that L′ is the physical cavity lengthcorresponding to the desired optical path length L, the two quantitiesbeing related by the equation L=nL′, where n is the index of refractionof the medium in the cavity.).

In practice it has been difficult and expensive to achieve the desireddegree of perfection because of the very narrow tolerances (in the orderof nanometers) required for the level of performance associated withextremely narrowband applications. U.S. Pat. Nos. 6,181,726 and6,215,802 disclosed several advances over the prior art whereby theuniformity of the etalon's optical length was improved. According to oneapproach described in the patents, all the spacers used to form theetalon are selected from a common local area of a spacer substrateproduced by standard-precision optical manufacturing techniques. It wasdiscovered that, as a result of this selection, the spacers tend to havesubstantially more uniform thickness and, therefore, they produce a moreuniform etalon cavity. According to another, complementary approach, anadditional spacer from the same local substrate area is used at thecenter of the etalon, thereby providing a correction to planedeformations produced by the optical contact of the peripheral spacerswith the etalon plates.

While the techniques described in these patents provide a significantimprovement over the etalons previously known in the art, they are verylabor-intensive and therefore expensive to practice. In addition, theresulting etalons, while more uniform in the optical length of thecavity, are not necessarily tuned to the precise desired wavelength.Copending U.S. Ser. No. 10/795,167 discloses a solution to this problembased on the use of counterbalanced forces applied to the etalonelements. This approach constitutes another significant advance in theart, but it does not address the problem of changes in performance (interms of center wavelength and FSR) produced by thermal variations.Therefore, there is still a need for an etalon structure that isrelatively insensitive to thermal effects and that produces extremelyaccurate tuning of the optical length of the etalon cavity. The presentinvention provides a solution to this remaining challenge that alsoproduces a greater range of angular acceptance and a mechanism forthermally tuning the etalon to a precise level of performance.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a temperature-stable and temperature-tunablecomposite etalon constructed by partially filling the cavity of anair-spaced etalon with a solid parallel-plate filler. The etalon platesare spaced apart in conventional manner using a plurality of spacers andthe resulting air gap is partially filled with an optically transmissivematerial (hereinafter referred to as a “filler plate”) having apredetermined thermal coefficient of expansion in relation to that ofthe spacers, thereby producing an etalon referred to as a “compositesolid-air-spaced etalon.” The filler plate is optically contacted to oneof the etalon mirror coatings inside the cavity and replaces part of theair gap. According to one aspect of the invention, the filler plate'scoefficient of expansion is chosen opposite in sign to that of thematerial of the spacers and this parameter, in conjunction with thethickness of the filler plate, may be judiciously selected so as toprovide a thermally stable etalon within a substantial range oftemperature operation.

According to another aspect of the invention, because the thermalbehavior of the composite etalon is predictable and quantifiable on thebasis of the thermal characteristics of the filler material, the fillerplate may alternatively be used to thermally tune the etalon to aprecise desired spectral performance. Since it is known that atemperature change causes a related change in the optical length of thecavity (and correspondingly a shift in the passband center wavelength),the presence of the filler plate can be advantageously utilized toaffect the thermal dependence of the optical cavity length. That is, thecoefficient of thermal expansion and the index of refraction of thefiller material are selected so as to produce the desired rate of changeof the optical length of the cavity as a function of temperature. Thus,the index of refraction and the thickness of the filler-plate can beused to manipulate the thermal response of the etalon, either increasingor decreasing the thermal response of a corresponding air-spaced etalon,as desirable for a particular application. In essence, these parametersprovide two additional degrees of freedom over the prior-art parametersutilized in the design of etalons.

According to yet another aspect of the invention, the filler plate ispreferably chosen to be significantly thicker than the remaining air gapin the etalon cavity. As a result of this configuration, while thepresence of the air gap permits spectral tunability of the etalon (bothby prior-art mechanical means and by thermal means, as disclosedherein), the fact that the filler plate occupies a large part of thecavity space increases the field of view of the etalon of the inventionin comparison with corresponding conventional air-spaced etalons.

Thus, the invention provides a composite etalon filter capable ofoperating at a single wavelength or at a plurality of wavelengths, withseveral advantageous characteristics produced simultaneously by itsnovel aspects. First, the spectral performance of the filter can bethermally tuned over a wider range than achievable with conventionaletalons; second, the angular acceptance of light by the filter (withinthe tolerances of the desired optical performance) is increased ascompared to that of conventional air-spaced etalons; finally, thefiller-plate material and its thickness can be selected to producesubstantial thermal stability within a predetermined range oftemperature operation.

Various other aspects and advantages of the invention will become clearfrom the description in the specification that follows and from thenovel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention consists of the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiments, and particularly pointed out in the claims. However, suchdrawings and descriptions disclose only some of the various ways inwhich the invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a conventional air-spaced etalon.

FIG. 2 is an illustration of a typical intensity-versus-wavelengthspectrum of the transmission beam produced by an etalon.

FIG. 3A is a schematic representation of an etalon according to theinvention shown in a side view wherein the etalon is partially rotatedaround its longitudinal axis to visibly display all spacers.

FIG. 3B is a cross-section of the etalon of FIG. 3A as seen from theplane defined by lines 3B in FIG. 3A.

FIG. 4 shows three graphs of the tuning range, as a function of fillingratio and expressed in units normalized to FSR, for three differentetalons according to the invention.

FIG. 5 shows three graphs of the thermal tuning rate, as a function offilling ratio and expressed in units normalized to FSR, for the samethree different etalons of FIG. 4.

FIG. 6 is a contour plot illustrating the spectral shift of thetransmission peak as a function of field of view and filling ratio forthe etalon of the invention (solid lines) as compared to conventionalair-spaced etalons (dashed lines).

FIG. 7A is a schematic representation in side view of another etalonaccording to the invention including an additional central foot spacerand wherein the etalon is partially rotated around its longitudinal axisto visibly display the peripheral spacers.

FIG. 7B is a cross-section of the etalon of FIG. 7A as seen from theplane defined by lines 7B in FIG. 7A.

FIG. 8A is a schematic representation in side view of another etalonaccording to the invention including a tubular ring spacer instead ofperipheral spacers.

FIG. 8B is a cross-section of the etalon of FIG. 8A as seen from theplane defined by lines 8B in FIG. 8A.

FIG. 9A is a schematic representation in side view of another etalonaccording to the invention including a central foot spacer in additionto the ring spacer of FIGS. 8A/B.

FIG. 9B is a cross-section of the etalon of FIG. 9A as seen from theplane defined by lines 9B in FIG. 9A.

DETAILED DESCRIPTION OF THE INVENTION

This invention is based on the general idea of introducing atransmissive filler material in the cavity of a conventional air-spacedetalon with thermal and optical characteristics designed to produce apredictable thermal behavior within a range of performance of interestfor a particular application. These parameters may be used to produce athermally tunable or a thermally stable etalon filter. In either case,the field of view of the filter is greatly increased by the presence ofthe filler plate within the etalon structure.

For the purposes of this disclosure, “angle of acceptance” and “field ofview” of an etalon filter are defined, as commonly accepted in the art,as the maximum angle of incidence within which the spectralcharacteristics of the etalon remain within predetermined acceptableboundaries. The term “spacer” refers to any structural component thatcreates a separation between the optical surfaces of an etalon, whetheror not the spacer is integral with another structure and whether or notit consists of a mechanism or a body of material. “Optical surface”refers to either of the two reflective surfaces constituting the etalonof the invention. For the purposes of claiming this invention, the term“medium,” used to describe the fluid of solid material between theoptical surfaces of an optical cavity, is also intended to includevacuum. Finally, “thermal stability” of an optical cavity is defined asa condition whereby a negligible change in the optical cavity length isobserved within a range of operating temperatures. Typical air-spacedetalons with spacers made of CLEARCERAM-HS and a cavity length of 400microns exhibit a variation in center wavelength of about 0.5 percent ofchannel spacing per degree centigrade. Comparable solid etalons exhibita greater temperature instability. Therefore, for the purposes of thisdisclosure any change in the length of an etalon cavity smaller thanabout 0.5 percent of channel spacing per degree C. is considered“negligible.”

Referring to the figures, wherein like parts are designated with thesame reference numerals and symbols, FIGS. 3A and 3B illustrate anembodiment of an etalon 30 according to the invention in side andsectioned views, respectively. The etalon 30 features two optical plates18 and 20 optically contacted to and separated by at least one spacer ora plurality of spacers S (three are illustrated in these figures).Plates 18,20 are usually about several centimeters in diameter, but canbe constructed in various dimensions depending on the application. Thethickness of the etalon plates may vary, but a preferred thickness isabout ⅙ of the plate diameter. Each of the etalon plates has a centralclear aperture 32 for transmitting light. The etalon's cavity 14 isdefined by high-reflectance coatings 34 and 36 placed on the interiorsurfaces of the etalon plates 18 and 20, respectively. The exteriorsurfaces 38,40 of the plates 18,20 may optionally also be coated in thearea of the clear aperture 32 with a thin-film coating to enhanceoptical performance. According to the invention, an additional opticalelement, the transmissive filler plate F, is optically contacted to thehigh-reflectance coating 36 of the etalon plate 20, thereby leaving anair gap G separating the filler plate from the reflective surface 34 ofthe front plate 18. Because the etalon's cavity is defined by thehigh-reflectance coatings 34 and 36, the optical length of the cavity isdefined by the sum of the optical thicknesses d_(F) and d_(G) of thefiller plate F and the air gap G, respectively. In the preferredembodiment of the etalon filter, the air gap is elected to be thinnerthan the filler plate.

According to one aspect of the invention, the materials and thethicknesses of the spacers and the filler plate are appropriately chosensuch that the rate of change of the optical length of the etalon cavitydue to the change of the spacers' thickness with temperature is eitherincreased or decreased by the combined effect of the correspondingchanges in the refractive index and thickness of the filler plate withtemperature. If the rate is increased to an operationally significantlevel, temperature changes can be used to tune the etalon in anefficient and predictable manner. During temperature tuning, thethickness of the air gap d_(G) varies as the difference between therespective thicknesses of the spacers (d_(s)) and the filler plate(d_(F)). In addition, the optical length is affected by thermal changesin the refractive index of the filler-plate material, as well, all ofwhich provide control parameters that can be used advantageously todesign an etalon filter with an optical length that is thermallyvariable according to a predetermined response. Accordingly, thisfeature allows the composite solid-air-spaced etalon 30 of the inventionto be precisely tuned spectrally by controlling ambient temperature.

According to another, alternative, aspect of the invention, if thethermal rate of change of the optical length of the cavity is decreasedto a negligible level, a temperature stable etalon filter is produced.Still with reference to FIG. 3A, the etalon filter of the inventionoperates as follows. Incident light I, propagating through the filteralong the etalon's main axis z at reference temperature T, traverses theoptical length L of the cavity, which is defined by the followingequation:L=(d _(s) −d _(F) −d _(c))·n _(G) +d _(F) ·n _(F),  (1)where d_(S), d_(F), d_(C) are the thicknesses of the spacers S, thefiller plate F, and the optical coatings 34,36, respectively; and n_(S),n_(F), n_(G) are the refractive indices of the spacers, the fillerplate, and the air gap G, respectively.

Taking the partial derivative of Equation 1 with respect to temperatureand recognizing the relatively very small change of n_(G) withtemperature, the following approximate equation is derived:$\begin{matrix}{\frac{\partial L}{\partial T} \approx {{d_{S}n_{G}\alpha_{S}} + {d_{F}\left\lbrack {{\left( {n_{F} - n_{G}} \right)\alpha_{F}} + \frac{\partial n_{F}}{\partial T}} \right\rbrack}}} & (2)\end{matrix}$wherein α_(s),α_(F) are the coefficients of thermal expansion of thespacers and the filler plate, respectively; and ∂an_(F)/∂T is thecoefficient of the filler plate's refractive index change withtemperature. From Equation 2, it is apparent that the material used toconstruct the filler plate F can be judiciously selected to increase ordecrease the rate at which the optical cavity length L varies withtemperature within a predetermined operating range, thereby providing apractical tuning mechanism or a means for achieving a thermally stableetalon. Furthermore, the filler plate's thickness d_(F) also provides aparameter that can be used to either increase or decrease the rate atwhich the optical length of the etalon varies with temperature. Forexample, an etalon (for DWDM application, 1550 nm wavelength, 0.8 nmchannel spacing) constructed in a 400-micron physical cavity length,with CLEARCERAM-HS spacers and a filler plate made of fused silica, wastunable at a rate that was less than 0.5% of the channel spacing perdegree C. for filling ratios up to 0.35.

As is well known in the art, when operating in converging light, theoptical performance of an etalon (such as peak wavelength, transmissionbandwidth, and peak transmission) changes as a function of the angle ofconvergence (see, for example, H. A. Macleod, Thin Film Optical Filters,IOP Pub., 3^(rd) ed., 2001). It is also well known that solid-spacedetalon filters accept light within a wider angle of convergence thanair-spaced etalons while maintaining optical performance within apredetermined tolerance window. Comparing two etalons 1 and 2, theirfields of view (equal to twice their corresponding acceptancesemi-angles θ₁ and θ₂, respectively) with analogous optical performancewithin the same tolerance window are proportional to the effectiverefractive indices n₁ and n₂ of the cavities comprising the respectiveetalons. Indeed, it can be shown that, when the etalon cavity isuniformly filled with several optical materials geometrically containedwithin plane surfaces that are parallel to the surfaces of the etalonplates, the overall effective refractive index of the cavity gap(hereinafter referred to as the “effective cavity index”) can be modeledas a weighted sum of the refractive indices involved, where the weightcoefficients are taken in proportion to the length of the cavityoccupied by materials in question. Accordingly, the effective indexn_(eff) of a cavity that is composed of i layers of materials withindices n_(i) and thicknesses l_(i) is calculated as $\begin{matrix}{n_{eff} = {\sum\limits_{i}\quad{\frac{l_{i}}{L^{\prime}}n_{i}}}} & (3)\end{matrix}$where L′ is the overall geometrical length of the etalon cavity.Therefore, partial filling the etalon gap with a filler-plate materialthat is optically denser than air produces an increased angle ofacceptance with respect to a corresponding conventional air-spacedetalon. It can also be shown that a larger filling ratio, defined forthe purposes of this invention as the ratio of filler-plate's thicknessd_(F) to that of the overall etalon gap L′=(d_(F)+d_(G)), produces awider field of view.

In a given interference order m, the etalon filter is known to transmita spectral band centered around a wavelength λ′ that satisfies thecondition 2L=mλ′. Thus, the spectral tuning of the etalon as a functionof temperature change ΔT is modeled as $\begin{matrix}{{\Delta\quad\lambda^{\prime}} = {\lambda^{\prime}\left( {1 + {\frac{\partial\lambda^{\prime}}{\partial T}\Delta\quad T}} \right)}} & (4)\end{matrix}$where Δλ′ is the spectral shift of the peak wavelength of the etalon'scharacteristic in order m produced by the temperature change ΔT. As aparticular example, an etalon according to the invention of FIG. 3A wasconstructed to operate at a wavelength of λ=656 nm. The etalon plates18,20 as well as the spacers S were made of fused silica with a thermalcoefficient of expansion (CTE) approximately 7.1*10−6 1/° C. Thethickness of spacers was approximately 400 μm, while the thickness ofthe filler plate F (NEX-C glass sold by Ohara Corp. of Japan, withCTE˜−0.2*10⁻⁶ 1/° C. and dn/dT˜18.2*10⁻⁶) was about 350 μm. An air gap Gof about 50 μm was left between the filler plate and the front etalonplate. Temperature tuning over a single free-spectral range of theetalon (FSR˜4 Angstrom) was accomplished over the range of about 35° C.at a rate of about 10.7 pm/° C. The effective cavity index of thisetalon at 20° C. equaled approximately 1.4799, and its acceptancesemi-angle (or half field-of-view) was about 48% wider than that of acorresponding air-spaced etalon with the same cavity length andmaterials.

In another example, the spacers were fabricated using NEX-C glass whilethe filler plate was made of fused silica (dn/dT˜5.5*10⁻⁶). The otherparameters remained the same. Tuning across about 0.75 of thefree-spectral range of the etalon was possible over approximately 60° C.of temperature range at a rate of about 5 pm/° C. In this case, theeffective cavity index was about 1.4025 and the acceptance angle of theetalon filter was about 40% greater than that of the comparableair-spaced etalon.

FIGS. 4 and 5 illustrate two temperature tuning characteristics (therange of thermal tuning and the thermal tuning rate, both expressed interms of units normalized to the free spectral range of the respectiveetalons) for three different etalons (A, B, and C) as functions of thefilling ratio. The etalons utilized NEX-C and CLEARCERAM-HS (both byOhara Corporation) as optical materials. Table 1 below summarizes thespecifications of each etalon. TABLE 1 Parameters of compositesolid-air-gap etalons used for FIGS. 4 and 5 Cavity Peak Filler SpacerSubstrate Length, Wavelength, Material Material Material um nm A FusedCLEARCERAM- Fused Silica 400 656 Silica HS B NEX-C Fused Silica FusedSilica 400 1550 C Fused CLEARCERAM- Fused Silica 400 1310 Silica HS

The tuning ranges shown in FIG. 4 were obtained over a 60 deg C.temperature span. As those skilled in the art would readily understand,while the thermal tuning rate of a given etalon constructed according tothe invention depends exclusively on its filling ratio, the overalltuning range for a given temperature span also depends on the physicallength of the etalon cavity, a thicker etalon cavity providing a widerrange of tuning. These properties are extremely useful because they makeit possible to tune the etalon filters of the invention across acomplete free spectral range within a temperature change that ispractically manageable as an operating parameter. This is of aparticular usefulness in many areas of technology, ranging from imagingand atmospheric observation to telecommunications. Fused silica, NEX-Cand CLEARCERAM-HS in various combinations and thicknesses have shown tobe good materials to achieve the thermal control that is at the heart ofthe invention.

FIG. 6 is a contour plot illustrating the spectral shift (measured inangstroms) of the transmission peak as a function of field of view andfilling ratio for the etalon of the invention (solid lines) as comparedto conventional air-spaced etalons (dashed lines). The solid lineillustrate various spectral shifts of the peak wavelength (0.3, 0.5 and1.0 angstroms) as a function of half FOV and filling ratio for etalon Dconstructed with the materials listed in Table 2 below. The dashed linesshow the corresponding performance of conventional air-spaced etalons Emade using materials also listed in Table 2. FIG. 6 also illustrates theincreased field of view (FOV) of a tunable composite solid-air-spacedetalon according to invention (etalon D) as compared to a conventionalair-spaced etalon (etalon E). As expected, the etalon of the inventionexhibited progressively greater angles of acceptance with higher fillingratios, while the field of view remained constant for the conventionalair-spaced etalon. For example, if the maximum allowable spectral shiftis 0.5 angstroms, the half FOV of the composite etalon E with a fillingratio of about 0.9 satisfying this requirement is 1.42 (see point 48 inFIG. 6). This represents an increase of about 42% with respect to theair-spaced etalon E satisfying the same requirement (represented by the0.5 dashed line). TABLE 2 Parameters of composite and air-gap etalonsused for FIG. 6 Cavity Peak Filler Spacer Substrate Length, Wavelength,Material Material Material um nm D Fused CLEARCERAM- Fused Silica 400656 Silica HS E Air CLEARCERAM- Fused Silica 400 656 HS

Another embodiment 50 of the etalon of the invention is shown in FIGS.7A and 7B in side and sectioned front views, respectively. Here, thefiller plate F has a central opening 52 (which preferably alsocorresponds to respective openings 54 in the coatings 34 and 36) wherean additional centrally located foot spacer S is positioned between theetalon plates 18,20 for enhanced performance. The optical performance ofthis embodiment is analogous to that described with respect to FIGS. 3Aand 3B.

An alternative embodiment 60, shown in FIGS. 8A and 8B in side andsectioned front views, respectively, provides a structure analogous tothat of FIGS. 3A/B wherein a ring-spacer S′ is used instead of multiplespacers S distributed around the periphery of the etalon plates, as iswell known in the art. The optical performance of this embodiment isanalogous to that described with respect to the embodiments of FIGS. 3Aand 3B as well.

Yet another embodiment 70, shown in FIGS. 9A and 9B, demonstrates ahybrid spacer structure composed of the ring-spacer S′ positioned alongthe perimeter of the etalon and a foot spacer S placed in the centralopening 52 of the filler plate F (which preferably also includescorresponding central openings 54 in the high-reflectance coatings 34and 36. The optical performance of this etalon is again analogous tothat described with respect to the other embodiments.

The invention has been shown and described with respect to certainpreferred principles, embodiments and features. It is understood thatthese embodiments are representative of the subject matter broadlycontemplated by the invention, and that the scope of the invention fullyencompasses other embodiments which may become useful in the art. Forexample, the etalon gap may be filled with multiple layers of fillerplates, or with matter other than air or vacuum, such as a liquid oranother gas possessing required optical characteristics. Similarly, itis understood that in practice the optical surface (or surfaces) of thefiller in contact with air would be coated with an antireflectivecoating.

Thus, while the invention has been shown and described in what arebelieved to be the most practical and preferred embodiments, it isrecognized that departures can be made therefrom within the scope of theinvention, which is therefore not to be limited to the details disclosedherein, but is to be accorded the full scope of the claims so as toembrace any and all equivalent apparatus and methods.

1. A method of increasing a field of view of an etalon filter thatincludes an optical cavity defined by two substantially parallel opticalsurfaces separated by at least one medium with a corresponding mediumindex of refraction, said index of refraction contributing to aneffective cavity index of refraction, the method comprising the step of:placing a transmissive filler plate within the optical cavity toestablish a new effective cavity index of refraction, wherein thetransmissive filler plate is made of a material having a higher index ofrefraction than said medium index of refraction, so that the neweffective cavity index of refraction is greater than said effectivecavity index of refraction.
 2. The method of claim 1, wherein each ofsaid optical surfaces is a surface of an optical plate.
 3. The method ofclaim 1, wherein each of said optical surfaces is a coated surface of anoptical plate.
 4. The method of claim 1, wherein said optical surfacesare separated by a spacer structure that includes a plurality of spacersperipherally distributed between the optical surfaces.
 5. The method ofclaim 4, further including a central foot spacer axially disposedbetween said optical surfaces through an aperture in said transmissivefiller plate.
 6. The method of claim 1, wherein said optical surfacesare separated by a spacer structure that includes a tubular ring spacerbetween the optical surfaces.
 7. The method of claim 6, furtherincluding a central foot spacer axially disposed between said opticalsurfaces through an aperture in said transmissive filler plate.
 8. Themethod of claim 1, wherein said transmissive filler plate is in opticalcontact with one of said two optical surfaces.
 9. The method of claim 1,wherein said transmissive filler plate has a surface coated with anantireflective coating.
 10. The method of claim 1, wherein said opticalsurfaces are separated by a spacer structure that is made of a spacermaterial selected from the group consisting of fused silica,CLEARCERAM-HS and NEX-C; and said transmissive filler plate is made of afiller material selected from the group consisting of fused silica,CLEARCERAM-HS and NEX-C.
 11. The method of claim 2, wherein each of saidoptical surfaces is a coated surface of an optical plate, and saidoptical surfaces are separated by a spacer structure that includes aplurality of spacers peripherally distributed between said opticalsurfaces.
 12. An optical cavity with a field of view increased accordingto the method of claim
 1. 13. An optical cavity with a field of viewincreased according to the method of claim
 2. 14. An optical cavity witha field of view increased according to the method of claim
 3. 15. Anoptical cavity with a field of view increased according to the method ofclaim
 4. 16. An optical cavity with a field of view increased accordingto the method of claim
 5. 17. An optical cavity with a field of viewincreased according to the method of claim
 6. 18. An optical cavity witha field of view increased according to the method of claim
 7. 19. Anoptical cavity with a field of view increased according to the method ofclaim
 8. 20. An optical cavity with a field of view increased accordingto the method of claim 9.